1. Field of the Invention
The present invention relates to an apparatus for supporting a workpiece. It is particularly, but not exclusively, concerned with supporting a semiconductor wafer within a vacuum system.
2. Summary of the Prior Art
In processing a semiconductor wafer, it is often necessary to carry out at least some of the processing at reduced pressure. Processes needing this include thin film sputtering, plasma etching, and chemical vapour deposition including plasma enhanced chemical vapour deposition. In an apparatus for processing a semiconductor wafer, the semiconductor wafer is supported on a platen within an enclosure, and then the pressure is reduced within that enclosure. It is important to ensure that there is no contamination of the wafer from e.g. the fabric of the platen. For this reason, it is standard practice to use a metal platen with a highly polished surface, so that the risk of contamination of the wafer is minimised.
When carrying out processing of a semiconductor wafer, it is usually necessary to achieve a specific wafer temperature rapidly, and to maintain that specific temperature during the processing operation. Techniques for controlling the temperature of the platen and of the enclosure are well established, but such techniques do not always achieve the result of satisfactory control of wafer temperature. In particular, there is normally high thermal resistance between the wafer and the platen and, although proposals have been made to overcome this problem by mechanically clamping the wafer to the platen, or by introducing gas between the wafer and the platen, such techniques have not been wholly successful. The introduction of gas also has the disadvantage of causing a stress on the wafer, and mechanical clamping may generate particles.